1. Field of the Invention
The present invention relates to an image reading-out apparatus, in particular, an image reading-out apparatus for exposing and scanning an original document to read out the image on the original document and thereby obtaining image data. Further, the present invention relates to a copying machine and a facsimile device which include the image reading-out apparatus. Furthermore, the present invention relates to a method of reading out an image.
2. Discussion of the Background
Hitherto, in the image reading-out apparatus for creating the image data from the image on the original document by use of the opto-electric conversion element such as CCD (Charge Coupled Device), the black-shading compensation has been performed for the image data.
Namely, the image data outputted from the CCD are composed of a signal component of dark current not changing in accordance with the light receiving amount from the CCD and another signal component changing in accordance with the same. Since the signal component effective for the image processing is only the signal component changing in accordance with the light receiving amount of the CCD, the black-shading compensation is performed by subtracting the signal component of the dark current (black level value) from the image data outputted by the CCD.
The black level value can be obtained line by line in average the output of the optical black (OPB) portion provided in a part of the CCD (tip end portion in the main scanning direction), for instance, before reading out the original document by use of the CCD. Moreover, the OPB portion in the CCD is, for instance, the part in which the surface of only the several pieces of pixels at the side of starting the scan in the main scanning direction, for instance, among the pixels of the optoelectric conversion elements arranged in a row of the CCD is vaporized with aluminum, and thereby the black level value can be obtained.
However, the background level of the original document is detected from the image data outputted from the CCD and the image data are changed in accordance with the background level of the original document on which the reference voltage of the analog-to-digital (A/D) converter at the time of the A/D conversion, and thereby the following of the black standard level detection at the black-shading compensation is delayed and the unevenness of the density occurs in the image in the case of adopting the member of removing the influence exerted on the background color of the original document from the image data. This is the troublesome matter to be solved.
Furthermore, in the system required the real-time property to perform line by line the black-shading compensation, since the number of the pixels of the optoelectric element which can be used in the OPB portion is limited, the unevenness occurs in the average value of the black level value in the image reading-out apparatus of the unpreferable S/N ratio. As the result, since the black-shading compensation differs per each line, even though the black level lines exist successively in the sub-scanning direction, the black data level differs per each line on the read-out image data and thereby the transverse stripe occurs on the image. This is also the troublesome matter to be solved.
One of the methods of solving the above matter is to improve the aforementioned S/N ratio. However, it is very difficult in consideration of the cost and technical matters. It can be thought to adopt the other method of eliminating the real time property to perform line by line the black-shading compensation as mentioned before. However, according to this method, the system employed hitherto has to be largely changed.
Heretofore, the background arts regarding the image reading-out apparatus and the copying machine and facsimile device both including the image reading-out apparatus have been described. However, according to such background arts, there exists no advantageous functional effect for improving the image reading-out apparatus.